This invention relates to an illumination system, and more particularly relates to a collimated-light illumination system suitable for use in color projection display systems employing one or more electro-optic light modulators.
Most color projection display systems employing one or more electro-optic light modulators, also employ a single-lamp white light illumination source, a first se of dichroic filters arranged to separate the white light from the source into primary (red, blue and green) color components, and a second set of dichroic filters to recombine the components after modulation.
The light modulators usually are active matrix liquid crystal display panels, either transmissive or reflective, and the light source is a high intensity discharge lamp with a small arc length. This small arc length results in a relatively compact illumination beam having a low etendue. Etendue is a term used in optics to define the geometrical limits of an illumination beam, in terms of both area and angular extent. The low etendue of the illumination beam enables the display panel to utilize a large portion of the beam despite the relatively small panel size (approximately 1.3 inches or less).
Considerable effort has been made to further reduce the size, and consequently the cost, of these complex display panels. As the size of such panels are reduced, the arc length of the lamp must also be reduced, in order to reduce the etendue of the illumination beam, thereby maintaining an adequate lumen output of the projection system.
Unfortunately, as arc length is reduced in size, the current density in the lamp electrodes increases until a practical limit is reached. Unfortunately, at the present state of the art of lamp technology, this practical limit dictates arc lengths which result in projected images of marginal brightness.
In the published abstract 6-242397 of Japanese Patent Application No. 5-29320, there is shown a projection display device having two light sources 20 and 21, whose respective illumination beams are condensed on opposed reflection surfaces 27 and 28 by condensing reflectors 31 and 32. The beams are thence reflected by surfaces 27 and 28 into a condensing lens system 33.
Due to the proximity of the reflection surfaces to each other and to the object point of the condensing lens system 33, the respective illumination beams are said to be made incident on lens system 33 in a state of superposition. However, due to the fact that the points of reflection of the two beams from the reflection surfaces are separated from one another, the state of superposition is not perfect, and the etendue of the superposed beams is increased.
Moreover, the need to locate the reflecting surfaces close to the object point of the condensing lens severely restricts the design freedom of such a system.